In many of the current applications for ultra-fast (i.e. repetition rates in the range of 1–100 kHz), ultra-short pulse lasers it is necessary or desirable to measure, and in some cases control, the energy of the laser beam that is incident on a target. For example, in laser refractive surgery the “laser-tissue” interaction within the cornea of the eye may be strongly energy dependent. It is preferable, therefore, to be able to measure the energy of the laser beam prior to the beam exiting the surgical laser system and entering the eye. Many of the current means for measuring the energy of pulsed laser beams are based on pyroelectric or thermal principles. Unfortunately, these methods are not able to accurately measure the energy of a single ultra-short laser pulse of a femtosecond laser beam, i.e. a beam with pulse durations in the range of about 1 fs to 100 ps and pulse repetition rates of several kilohertz. Thus, an alternative method is needed to measure the energy of ultra-fast, ultra-short laser pulse.
One possibility for measuring the energy of an ultra-fast, ultra-short laser pulse is to define the measurement function in terms of an equivalent electrical circuit. Using this approach, it would be possible to integrate a voltage output of the circuit over time to yield a value proportional to the input laser energy. Considering this approach in greater detail, a photodiode can be used to detect the individual pulses of a laser beam. When light corresponding to the operational wavelength of the photodiode is detected, the light is absorbed by the photodiode and charge carriers are separated within the photodiode. It is well known that the amount of separated charge carriers, which is actually the charge, is directly proportional to the energy of the incident laser pulse. If both ends of the photodiode are electrically connected, the separated charge carriers will equalize and generate a current. Notably, the integral of this current over time is proportional to the amount of the separated charge carriers. It follows, therefore, that the integral of a voltage generated by the current is also proportional to the amount of separated charge carriers. Importantly, the value of this integral is the output value, and it is proportional to the energy of the incident laser pulse. With regard to the integration of the voltage discussed above, a fast analog integrator is used to calculate the integral. The integration process can be started either before, during, or very shortly after the laser pulse reaches the photodiode.
There are several advantages to using a photodiode circuit to define and quantify the pulse energy in terms of the integral of the voltage. It should be noted, however, that many photodiodes comprise “peak” value detectors. Unfortunately, this type of detector is not capable of measuring ultra-short pulses at very high repetition rates. Nonetheless, it should be possible with certain photodiodes well known in the pertinent art to measure the energy of a single ultra-short pulse. In this context the measurement is actually the integration of the impulse response of the photodiode. An advantage of integrating the impulse response is that the measurement is not dependent on the pulse duration or number of pulses. A further advantage is that fluctuations of the photodiode's capacity do not affect the ultimate measurement. Also, pulse to pulse energy measurements at high repetition rates can be achieved.
In addition to quantifying the energy of a laser pulse, the measured energy data can be used as feedback into a control loop for controlling the energy of the laser beam as well. With regard to medical applications, such as laser refractive surgery, both system efficiency and patient safety drive the need for such positive control.
In light of the above, it is an object of the present invention to provide a system and method for measuring the energy of a laser beam. Another object of the present invention is to provide a system and method for measuring the energy of a single ultra-fast, ultra-short pulse of a laser beam, wherein the laser beam comprises a plurality of pulses at very high repetition rates, in the range of 1–100 kHz. Yet another object of the present invention is to provide a system and method for measuring the energy of an ultra-fast, ultra-short pulse of a laser beam for controlling the laser energy incident on a target. Still another object of the present invention is to provide a system and method for measuring the energy of an ultra-fast, ultra-short pulse of a laser beam that is easy to use, relatively simple to manufacture, and comparatively cost effective.